Composite mold with fugitive metal backup

ABSTRACT

Composite mold for use in casting a metal or alloy includes an inner mold region that includes a mold cavity and a fugitive metallic outer backup mold region residing on the inner mold region, wherein the metallic material of the backup mold region has such a melting temperature that the backup mold region melts from the inner mold region after the molten metal or alloy is cast and at least partially solidified in the mold. The inner mold region can comprise a non-metallic refractory or ceramic shell mold, while the backup mold region can comprise tin or other relatively low melting point metal or alloy.

FIELD OF THE INVENTION

The invention relates to casting of metals and alloys and, inparticular, to a composite casting mold having an inner mold region andfugitive metallic outer backup mold region on the inner mold region foruse in casting high temperature melting metals and alloys.

BACKGROUND OF THE INVENTION

In the well known “lost wax” process of investment casting, a fugitiveor disposable pattern, such as a wax, polystyrene or other commonly usedfugitive pattern material, of the article to be cast is made byinjection molding a fluid pattern material in a die corresponding to theconfiguration of the article to be cast. That is, the fugitive patternis a replica of the article to be cast. In high production commercialinvestment casting operations, a plurality of fugitive patternstypically are attached to a central fugitive sprue and pour cup to forma gang or cluster pattern assembly. The pattern assembly then isinvested in a ceramic shell mold by repeatedly dipping the pattern in aceramic slurry having ceramic flour carried in a liquid binder, drainingexcess slurry, stuccoing the slurry layer while it is wet with coarserceramic particles or stucco, and then drying in air or controlledatmosphere until a desired thickness of a ceramic shell mold is built-upon the pattern. The initial ceramic slurry and stucco layers (e.g. theinitial several layers) form what is called a facecoat of the shell moldfor contacting the molten metal or alloy to be cast. The numerous outerbackup layers subsequently applied on the facecoat are selected toimpart sufficient strength to the mold to withstand casting of moltenmetal or alloy in the mold. A conventional ceramic shell mold can haveten or more facecoat and backup layers. The build-up of a ceramic shellmold of substantial thickness to withstand casting of molten metal oralloy in the mold requires considerable time and processing steps.

Once a shell mold of desired wall thickness is built up on the patternassembly, the pattern assembly is removed from the green shell moldtypically by a thermal treatment to selectively melt out the patternassembly, leaving a ceramic shell mold having one or more mold cavitieswith the shape of each fugitive pattern. One common pattern removaltechnique involves subjecting the green shell mold/pattern assembly to aflash dewaxing step where the green shell mold/pattern assembly isplaced in an oven at elevated temperature to rapidly melt the waxpattern from the green shell mold. Another pattern removal techniqueinvolves positioning the green shell mold/pattern assembly in a steamautoclave where steam at elevated temperature and pressure is used torapidly melt the pattern from the green shell mold. Following patternremoval, the shell mold is fired at elevated temperature to removepattern residue and to develop appropriate mold strength for casting amolten metal or alloy. Both the investment casting process and the lostwax shell mold building process are well known, for example, as isapparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751 aswell as numerous other patents.

The ceramic shell mold typically is cast with molten metal or alloy bypouring the molten material into a funnel-shaped pour cup of the shellmold and flowing the molten material by gravity down a sprue channelthrough gates and into the mold cavities. The molten metal or alloysolidifies in the mold to form the desired cast articles in the moldcavities. That is, the cast articles assume the shape of the moldcavities, which have the shape of the initial fugitive patterns. Thecast articles are connected to solidified gates, sprue and pouring cup.The ceramic shell mold then is removed, and the cast articles are cut orotherwise separated from the solidified gates and subjected to one ormore finishing and inspecting operations before being shipped to acustomer. The above described lost wax investment casting process is inwidespread use in casting components for use in gas turbine enginecomponents, airframes, vehicles, internal combustion engines, andnumerous other applications.

SUMMARY OF THE INVENTION

The present invention provides a composite mold for casting a metal oralloy, especially a high temperature melting metal or alloy, wherein themold includes an inner mold region having a mold cavity and a fugitivemetallic (metal or alloy) outer backup mold region residing on the innermold region, wherein the metallic material of the backup mold region hassuch a melting temperature that the backup mold region melts from theinner mold region after the molten metal or alloy is cast and at leastpartially solidified.

In an illustrative embodiment of the invention, the inner mold regioncomprises a relatively thin non-metallic refractory or ceramic shellmold.

In another illustrative embodiment of the invention, the outer backupmold region comprises tin or other metal or alloy that melts at atemperature of about 1300 degrees F. or below that extracts heat fromthe inner mold region after the molten metal or alloy is introduced intothe mold. The outer backup mold region can be cast and solidifiedin-situ on the inner mold region or otherwise formed on the inner moldregion.

The present invention also provides a method of casting a metal or alloycomprising the steps of introducing a molten metal or alloy into a moldcavity of a composite mold having an inner mold region that includes themold cavity and a fugitive metallic outer backup mold region residing onthe inner mold region, and melting the backup mold region from the moldafter the molten metal or alloy is cast and at least partiallysolidified. The molten metal or alloy can be introduced by any number ofknown gravity casting techniques or under pressure into the mold cavity;for example, by die casting or pressure casting into the mold cavity.

In an illustrative method embodiment of the invention, a method ofcasting titanium aluminide comprises introducing molten titaniumaluminide into a mold cavity of a composite mold having a relativelythin-wall ceramic inner shell mold region that includes the mold cavityand a fugitive metallic outer backup mold region residing on the innermold region, and melting the backup mold region away from the innershell mold region after the molten titanium aluminide is cast and atleast partially solidified in the mold. Melting of the outer backup moldregion leaves a thin ceramic shell mold region which does not overstressthe cast component as it cools, thereby avoiding hot cracking of thecast component.

The present invention further provides a method of making a casting moldcomprising the steps of forming a refractory or ceramic inner shell moldregion and forming a fugitive metallic outer backup mold region on theinner shell mold region. The outer backup mold region can be cast andsolidified in-situ on the inner shell mold region.

The above features of the present invention will become more readilyapparent from the following drawings taken with the following detaileddescription of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a ceramic inner shell mold for use inpractice of an embodiment of the invention.

FIG. 2A is a perspective view of the ceramic shell mold of FIG. 1 havingits sprue and mold section positioned in a mold. FIG. 2B is aperspective view of a composite mold including the inner ceramic shellmold region of FIG. 1 encased in a fugitive metallic outer backup moldregion.

FIG. 3 is a schematic side sectional view of a casting machine forpracticing a method embodiment of the invention with the vacuum chambershown broken away.

FIG. 4 is a perspective view of a portion of the casting machine havinga plurality of composite molds of FIG. 2 positioned to communicate to agating system of a die platen.

FIG. 5 is a perspective view of cast composite mold and castturbocharger wheels connected to gating after removal from the castingmachine.

FIG. 6 is a perspective view of a ceramic investment shell mold having asprue portion and mold-cavity portion with the mold-cavity-portion in acontainer in which a metallic backup material is to be cast andsolidified.

FIG. 7 is a perspective view of a composite mold formed after metallicmold backup material is cast and solidified in the container.

FIG. 8 is a schematic side sectional view of another casting machine forpracticing another method embodiment of the invention with the vacuumchamber shown broken away.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a composite mold for casting a metal oralloy, especially high temperature melting metals or alloys such asincluding, but not limited to, nickel based superalloys, cobalt basedsuperalloys, titanium alloys, titanium aluminide and other intermetallicalloys or compounds where the temperature of the molten metal or alloybeing introduced into the casting mold is quite high. For example,nickel base superalloys typically are cast at temperatures of 2300degrees F. and above. Titanium aluminide, such as gamma TiAl compound,typically are cast at temperatures of 2900 degrees F. and above. Theinvention is not limited to casting such high temperature melting metalsand alloys and can be practiced to cast any molten metal or alloy.

The composite mold of the invention includes an inner mold region havinga mold cavity and a fugitive metallic outer backup mold region residingon the inner mold region. The metallic material of the backup moldregion has such a melting temperature that the backup mold region meltsaway from the inner mold region after the molten metal or alloy is castand at least partially solidified. The metallic outer backup mold regioncomprises a relatively lower temperature melting metal or alloy materialcompared the metal or alloy to be cast to this end.

An illustrative inner mold region comprises a relatively thin ceramicshell mold formed by the “lost wax” process, although any mold makingprocess can be used in practice of the invention. The thickness of theinner shell mold region can be much less than a conventional “lost wax”shell mold since the inner mold region is structurally supported by themetallic outer backup mold region during casting and solidification ofthe molten metal or alloy in the mold. For purposes of illustration andnot limitation, a ceramic shell mold can have a mold wall thickness ofabout 0.1 inch or less, such as from about 0.04 inch to about 0.10 inch,and more particularly about 0.075 inch for an exemplary mold wallthickness.

The fugitive metallic outer backup mold region is selected to melt at atemperature substantially less than the metal or alloy to be cast in themold so that heat from the metal or alloy cast in the mold istransferred through the inner mold region and melts the outer backupmold region away from the inner mold region after the molten metal oralloy is cast and at least partially solidified. For purposes ofillustration and not limitation, the backup mold region comprises ametal or alloy that melts at a temperature of 1300 degrees F. or below.To this end, the outer backup mold region comprises a metallic materialincluding, but not limited to, tin, indium, bismuth, lead, aluminum, orzinc, or alloys thereof one with another or with other metals. Forexample, tin has a melting temperature of about 450 degrees F., indiumhas a melting temperature of about 312 degrees F., lead has a meltingtemperature of about 621 F, bismuth has a melting temperature of about520 degrees F., aluminum has a melting temperature of about 1220 degreesF., and zinc has a melting temperature of about 786 degrees F.

The outer backup mold region can be formed on the inner mold region byany appropriate process. For purposes of illustration and notlimitation, molten outer backup material can be cast and solidifiedin-situ on the inner mold region. Alternately, the outer backup moldregion can be formed by spraying molten backup material on the innermold region or attaching a previously formed backup region to the thinshell.

Referring to FIG. 1, for purposes of illustration and not limitation, athin-wall ceramic inner shell mold region 10′ is shown formed by the“lost wax” process to include a pour cup 10 a′ having a solid handlingcollar 10 b′, a sprue or runner 10 c′, and a mold section 10 d′ where awax or other fugitive pattern is repeatedly dipped in ceramic slurry,drained of excess ceramic slurry, and stuccoed with coarse ceramicstucco particles, and dried, until the desired wall thickness isbuilt-up. Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751 as well asnumerous other patents describe the “lost wax” process for making ashell mold.

In FIG. 1, the mold section 10 d′ has a turbocharger wheel-shape havinga plurality of vane-shaped mold sections 10 e′ disposed about theperiphery a central hub mold section 10 f′. The mold section 10 d′ has athin wall 10 w′ with a relatively small wall thickness as describedabove to define turbocharger wheel-shape mold cavity MC′in the moldsections 10 e′ and 10 f′. The pour cup 10 a′, collar 10 b′, and sprue orrunner 10′ can have a larger wall thickness than that of the moldsection 10 d′. The larger wall thickness of the pour cup 10 a′, collar10 b′, and sprue or runner 10 c′ is provided by using a preformedceramic or metal shape in those areas or by adding additional layers ofdipped ceramic in those areas.

For purposes of illustration and not limitation, the wall thickness ofthe mold section 10 d′ typically is in the range of 0.04 to 0.10 inch.Such thin wall typically comprises from 3 to 5 layers of facecoat plusstucco, which is substantially less than the number of layers employedto make a conventional “lost wax” mold.

The ceramic shell mold can be formed of one or more appropriate ceramicor non-metallic refractory materials, or combinations thereof, such asincluding but not limited to zirconia, silica, alumina, and graphite (asa non-metallic refractory) selected in dependence on the particularmolten metal or alloy to be cast in the mold 10′. Typically, the ceramicor refractory is selected so as to be substantially non-reactive withthe molten metal or alloy to be cast in the mold, although in somecasting applications reaction of the molten metal or alloy and theceramic or refractory inner mold region can be tolerated or desired. Asdescribed in the Example below, when the molten alloy to be castcomprises titanium aluminide, such as gamma TiAl, the ceramic innershell mold region 10′ can comprise aluminia, silica, yttria, or otherceramics.

After the ceramic inner shell mold region 10′ is formed, the fugitivemetallic outer backup mold region 20′ is formed thereon. Oneillustrative embodiment of the invention shown in FIG. 2A involvesplacing the sprue or runner 10 c′ and the mold section 10 d′ of theshell mold region 10′ in a cylindrical or other shaped metal mold 31′and introducing molten backup material (e.g. molten tin) in the moldcavity MM defined between the inner mold region 10′ and the mold 31′ toa level just below collar 10 b′ and solidifying the molten backupmaterial in-situ on the inner shell mold region 10′.

The mold 31′ then is removed to leave the composite mold 50′ pursuant toan illustrative embodiment of the invention comprising the inner moldregion 10′ and the fugitive metallic outer backup mold region 20′ on theinner mold region, FIG. 2B.

The outer backup mold region 20′ is relatively thick compared to theinner mold region 10′. For purposes of illustration and not limitation,the wall thickness of the outer backup mold region 20′ typically is inthe range of 0.15 to 0.4 inch when applied on the above-described thinwall inner mold region 10′ made by the “lost wax” process, although thethickness of the outer backup mold region 20′ may vary from location tolocation depending on the exterior configuration of the inner moldregion 10′. The invention is not limited to any particular thickness ofthe outer backup mold region 20′ so long as the outer backup mold regionforms a support body that is capable of supporting the inner mold regionand molten metal or alloy in the mold cavity and permits the inner moldregion 10′ to contain the molten metal or alloy when introduced in themold cavity under pressure.

The composite mold of the invention can be used in practice of variouscasting processes. For example, the composite mold is especially usefulin pressure casting of metals or alloys, especially high meltingtemperature metals or alloys wherein the molten metal or alloy isinjected or poured under superambient pressure into the composite mold.A modified die casting method is described in more detail below forpurposes of illustration and not limitation of the invention. Otherpressure casting processes include, but are not limited to, squeezecasting and centrifugal casting. The invention is also applicable totypical non-pressure casting such as gravity casting.

Regardless of the casting process employed, molten metal or alloy isintroduced into the mold cavity of the composite mold 50′ and solidifiedto form a cast component. After the molten metal or alloy is at leastpartially solidified in the mold, the fugitive metallic outer moldregion 20′ is designed to melt and drain away from the inner mold region10′ as result of extraction through the inner mold region 10′ of heatfrom the metal or alloy cast into the mold. The molten metal or alloy inthe mold 50′ is at least partially solidified to an extent that the thinwall inner mold region 10′ is capable of confining the metal or alloy inthe desired shape after the outer backup mold region 20′ is melted anddrained away. Typically, the molten metal or alloy is substantiallysolidified before the outer backup mold region 20′ begins to melt anddrain away from the inner mold region 10′.

The composite mold is especially useful for casting metals or alloysthat are sensitive to stress during cooling in the mold after casting.For example, some alloys, such as gamma TiAl intermetallic compound, areweak and brittle at temperatures encountered during cooling in the moldto ambient temperature. A thick wall conventional ceramic shell mold canbe strong enough to overstress the casting during casting, leading tocracking of the casting in the mold during cooling.

In contrast, the composite mold 50′ includes the inner mold region 10′to contain the molten metal or alloy and the outer backup mold region20′ that supports the inner mold region 10′ during casting and thatsubsequently melts and drains away from the inner mold region 10′ afterthe molten metal or alloy is at least partially solidified. This leavesthe at least partially solidified metal or alloy confined in the innermold region 10′, which lacks sufficient strength to overstress the castcomponent therein.

Moreover, the composite mold promotes rapid cooling of the molten metalor alloy in the mold cavity from the casting temperature to the metalbackup melting temperature. As is known, the cooling rate of the moltenmetal or alloy after casting affects many properties of the casting suchas grain size. The thin wall ceramic inner shell mold region 10′described above is less of a barrier to heat flow than a conventionalthick wall ceramic shell mold. The relatively cooler outer backup moldregion 20′ acts as a chill or heat sink to extract heat from the moltenor solidifying metal or alloy in the mold cavity. In addition, as thebackup mold region 20′ melts, its heat of fusion is extracted from themolten or solidifying metal or alloy in the mold cavity.

For purposes of illustration and not limitation, FIGS. 3 and 4illustrate placement of composite molds 50′ pursuant to the aboveillustrative embodiment of the invention in a modified die castingmachine. The die casting machine is shown comprising a base 11 whichincludes a reservoir (not shown) therein for hydraulic fluid that isused by hydraulic actuator 12 to move the movable die platen 16 relativeto the fixed (stationary) die platen 14 to open and close the dieplatens 14, 16. The platen 16 is disposed for movement on stationaryguide rods or bushings 18. A die platen clamping linkage mechanism (notshown) is connected to the movable die platen 16 in conventional mannernot considered part of the present invention.

The die casting apparatus also comprises a tubular, horizontal shotsleeve 24 having intermediate section that is received in the stationarydie platen 14 and a mold-receiving member or plate 30 fastened to theplaten 14 by bolts, clamps, and other fastening means. The shot sleeve24 extends into a vacuum melting chamber 40 where the metal or alloy tobe cast is melted under high vacuum conditions, such as less than 100microns, in the event an oxygen reactive metal or alloy, such astitanium alloy, titanium aluminide alloy, superalloy, etc., is to be diecast.

The vacuum chamber 40 is defined by a vacuum housing wall 42 thatextends about and encompasses or surrounds the charging end section 24 aof the shot sleeve 24 and the plunger hydraulic actuator 25 having ram25 a. The chamber wall 42 is vacuum tight sealed about the stationary,horizontal shot sleeve and plunger support members 44. The vacuumchamber 40 is evacuated by a conventional vacuum pump P connected to thechamber 40. The base 10 rests on a concrete floor or other suitablesupport.

A cylindrical plunger 27 is disposed in the cylindrical bore of the shotsleeve 24 for movement by ram 25 a between a start injection positionlocated to the left of a melt entry or inlet 50 in FIG. 3 and a finishinjection position proximate mold receiving member or plate 30. The meltinlet 50 comprises a melt receiving vessel 52 mounted on the shot sleeve24. The melt receiving vessel 52 is disposed beneath a melting crucible54 to receive a charge of molten metal or alloy therefrom for diecasting. The invention is not limited to a hydraulic plunger as a meansfor introducing the molten metallic material under superambient pressurein the mold 50′. For example, superambient gas pressure may be appliedat the end of the shot sleeve with or without the plunger present forintroducing the molten metallic material under pressure into the mold50′.

The melting crucible 54 may be an induction skull crucible comprisingcopper segments in which a charge of solid metal or alloy to be die castis melted. The charge of solid metal or alloy can be positioned in thecrucible 54 before a vacuum is established in chamber 40 and melted byenergization of induction coils 56 after the vacuum is established.Alternately, the solid metal or alloy charge can be charged into thecrucible 54 in evacuated chamber 40 via a vacuum port (not shown) andmelted by energization of induction coils 56. Known ceramic orrefractory lined crucibles also can be used in practicing the presentinvention. Any melting method such as arc melting, electron beammelting, and others may be employed in practice of the invention. Thecrucible 54 can be tilted to pour the molten metal or alloy charge intothe melt receiving vessel 52, which is communicated to the shot sleeve24 via an opening 58 in the shot sleeve wall. The molten metal or alloycharge is introduced through opening 58 into the shot sleeve 24 in frontof the plunger 27.

The plunger 27 is moved from the start injection position to the finishinjection position by conventional hydraulic actuator 25. Typical radialclearances between the shot sleeve 24 and the plunger 27 are in therange of 0.001 to 0.008 inch.

A die casting machine having the features described above is disclosedin U.S. Pat. No. 6,070,643 and in copending patent application Ser. No.11/311,433 of common assignee herewith, the teachings of both of whichare incorporated herein by reference.

The die casting machine 10 is modified or adapted to cast a moltenmetallic material under hydraulic pressure into one or more evacuatedcomposite molds 50′ described above.

Referring to FIGS. 1-4, pursuant to one embodiment of the invention,mold-receiving member 29 is fastened to platen 16 and is adapted to matewith mold-receiving member 30 that is fastened to platen 14 via plate 15to form a chamber C for receiving the composite molds 50′ and a moldgating system 35 therebetween when the members 29, 30 are abutted at avertical parting plane. The gating system is formed by machined,replaceable gating inserts 40, 42 that are received in respectivemembers 29, 30 and that are coplanar at their outermost surfaces withthose of the respective members 29, 30 in which they are received. Whenthe members 29, 30 are abutted at the parting plane, the gating insertsform a gating system that comprises runners R that communicate with acommon passage CP that communicates with the end of the shot sleeve 24.A respective runner R extends from the passage CP to a respective mold50′. The members 29, 30 as well as inserts 40, 42 typically are steel orother suitable permanent metal or alloy (metallic material) and aremounted on or connected to respective platens 14, 16 of the die castingmachine.

An O-ring vacuum seal S1 is provided between the members 29, 30 forestablishing a vacuum tight seal therebetween, FIG. 3. The vacuum sealS1 extends about and surrounds the gating system 35.

The composite molds 50′ are shown positioned in the chamber C with theirpour cups 10 a′ residing in complementary configured cylindrical shapedrecesses 41 a on a shelf or ledge 41 forming the bottom wall of thechamber C when the members 29, 30 are abutted at the parting plane.One-half of the ledge or shelf 41 as well as each recess 41 a is shownformed on the member 30 and the other half is formed on member 29. Themolds 50′ thereby are positioned vertically inverted such that theirrear closed ends 10 g′ face upwardly.

The end surface of each mold pour cup 10 a′ sealingly engages the shelfor ledge 41 in the recesses 41 a to prevent molten material from leakingout at the interface when the mold is clamped or pressed on the ledge asdescribed below. A flat seal or gasket optionally may be used if neededbetween the pour cup end surface and the ledge 41 to this end. The molds50′ are positioned relative to the runners R using a fixed positioningplate 51 having slots 51 a formed between fingers 51 b. Each mold 50′ isinserted in a respective slot 51 a with the adjacent fingers 51 b beingreceived in the mold positioning groove G′ formed between the pour cup10 a′ and collar 10 b′. One half of each mold pour cup 10 a′ thereby ispositioned to straddle a respective runner R to receive molten materialtherefrom. The positioning plate 51 is fixedly fastened to one of themembers 29, 30 in a horizontal orientation such that the fingers 51 bare received in the facing other of the members 29, 30 overlying theshelf or ledge of that member.

When so positioned, the pour cup 10 a′ and the sprue passage 10 c′ ofeach composite mold 50′ communicates to the shot sleeve 24 via thegating system for receiving molten metallic material from the shotsleeve 24 as pushed by the plunger 27. The shot sleeve 24 is sealinglyreceived in the member 30.

Each mold 50′ is supported in position in the chamber C against upwardforce of molten metallic material introduced into the mold via thegating system. For example, the upwardly facing end 10 g′ of each mold50′ is abutted by a respective support plate 60. Support plates 60 areconnected to shafts 62 each of which is mounted on a hinge 64 such thatthe plates 60 can be brought into position to abut the closed ends ofthe molds after they are positioned in positioning plate 51. The supportplates 60 are pressed gently toward the end of the molds 50′ by a mainshaft 66 connected to a respective hinge 64 and a pressing device 67,such as a spring, pneumatic cylinder, hydraulic cylinder, and/ormechanical clamp, to bias the shafts 66 downwardly.

The vacuum chamber 40 then is evacuated to a suitable level for meltingthe particular charge (e.g. less than 100 microns for titanium alloyssuch as Ti-6A1-4V alloy and titanium aluminide such as TiAl) by vacuumpump P. The composite molds 50′ in chamber C are concurrently evacuatedto the same vacuum level through the connection to the vacuum meltingchamber 40 via the shot sleeve 24 and by virtue of being isolated fromsurrounding ambient air atmosphere by the vacuum seal S1 between members29, 30. Optionally or in addition, the molds 50′ can be evacuated usinga separate vacuum conduits or lines communicated to the mold interior.

The composite molds 50′ typically are at ambient (room) temperature whenthey are placed in the chamber C. Alternately, the molds 50′ can bepreheated to a suitable elevated temperature before being placed in thechamber C. Still further, heaters (not shown) can be provided in thechamber C to heat or maintain the temperature of the molds 50′.

The solid charge of the metal or alloy in crucible 54 is melted byenergizing induction coil 56, the melt then is poured under vacuum intothe shot sleeve 24 via the melt inlet 50 with the plunger 27 initiallypositioned at the start injection position of FIG. 3. The molten metalor alloy is poured into the shot sleeve 24 and resides therein for apreselected dwell time to insure that no molten metal gets behind theplunger 27. The melt can be poured directly from the crucible 54 viainlet 50 into the shot sleeve 24, thereby reducing time and metalcooling before injection can begin.

The plunger 27 then is advanced in the shot sleeve 24 by actuator 25 toinject the molten metal or alloy under hydraulic pressure through thegating system and through the mold pour cup 10 a′ and sprue 10 c′ intothe mold cavity MC′ of each composite mold 50′. The plunger 27 isadvanced by a hydraulic system described in copending patent applicationSer. No. 11/311,433 incorporated herein by reference to this end.

The molten metal or alloy is forced at velocities, such as 10-120 inchesper second for titanium alloys and titanium aluminides, down the shotsleeve 24 and into the evacuated molds 50′.

After the molten metal or alloy is at least partially solidified,typically mostly solidified, in the molds 50′, the fugitive metallicouter mold region 20′ melts and drains away from the inner mold region10′ to a collection chamber or other region of the die designed tocollect the fugitive alloy as result of extraction of heat from the castand solidified metal or alloy through the inner mold region 10′.

After the molten metal or alloy is at least partially solidified,typically mostly solidified, the members 29, 30 are opened by movementof platen 16 away from platen 14 within a typical time period that canrange from 5 to 30 seconds following injection to provide enough timefor the molten metal or alloy to form at least a solidified surface onthe cast component (s) in the inner mold regions 10′. The metallicmaterial solidified in the inner mold regions 10′ typically issubstantially solidified by the time the inner mold regions 10′ areremoved from the chamber C. The inner mold regions 10′ and connectedsolidified runner 60′ then are removed from the chamber C andtransported to a demolding station where the inner mold regions 10′ areremoved from the cast component by conventional techniques forming nopart of the invention. The solidified runner 60′ can be removed beforeor after the inner mold regions 10′ are removed. FIG. 5 shows an innermold region 10′ and connected runner 60′ on the right hand side of thatfigure and turbocharger wheel castings 100′ on the left hand side ofthat figure after the inner mold regions 10′ have been removed. Thecastings then can be inspected visually and by techniques according tocustomer requirements.

In pressure casting titanium alloys, titanium aluminide, nickel basesuperalloys, and cobalt based superalloys, the shot sleeve 24 contactingthe molten metal or alloy can be made of an iron based material, such asH-13 tool steel, or a refractory material such as based on Mo alloy, Walloy, or TZM alloy, ceramic material such as alumina, or combinationsthereof that are compatible with the metal or alloy being melted and diecast. The forward plunger tip 27 a can comprise a permanent oralternately a disposable tip that is thrown away after each molten metalor alloy charge is injected in the investment mold 50′. A plunger tipcan comprise a copper based alloy such as a copper-beryllium alloy, orsteel, graphite, or other appropriate material.

The particular casting parameters employed to cast a component willdepend upon several factors including mold size, gating, pour weight,and the strength of the composite mold to the melt injection pressuresinvolved. The injection pressure is selected to retain the molds 50′intact (no mold cracking or bursting under pressure) while achieving asatisfactory fill of the mold cavity regions. The nominal weight ofmetal or alloy in the crucible 54 depends on the mold size and thenumber of components to be die cast in the mold.

The following EXAMPLE is offered to further illustrate the inventionwithout limiting it.

EXAMPLE 1

Turbocharger wheels have been successfully pressure cast of gammatitanium aluminide (TiAl) alloys (melting temperature of greater than2800 F degrees F.) in composite molds of the type shown in FIG. 2B usinga casting machine of the type shown in FIG. 3. The composite moldscomprised an inner mold region made of four dips of a wax pattern inaluminia and silica based ceramic slurry followed by stuccoing and airdrying to provide a wall thickness of about 0.075 inch. An outer moldregion of tin was applied to the inner mold region by casting to athickness of greater than 0.1 inch such that the molds had a shapesimilar to that shown in FIG. 2B.

In general, the turbocharger wheels were made using casting parametersin the following ranges: melt injection pressure settings: 400-1800 psi,melt injection velocities (plunger speed): 10-120 in/sec; TiAl meltsuperheat: 0 to 75 degrees F.; no mold preheat; shot sleeve length anddiameter: 17.38 inches and 2.80 inches; and limit switch 80 a set todump plunger fluid pressure when the plunger 27 carrying the switchactuator 80 b is about 0.75 inch from its final injection position. Themelted tin previously forming the outer backup mold region melted anddrained from the inner mold regions by gravity for reuse.

Referring to FIGS. 6-7, for purposes of further illustration and notlimitation, a thin-wall ceramic inner shell mold region 10″ is shownformed by the “lost wax” process to include a pour cup 10 a″ having asolid collar 10 b″, a sprue or runner 10 c″, and a mold section 10 d″for use in a different casting machine shown in FIG. 8.

In FIGS. 6-7, the mold section 10 d″ has a turbocharger wheel-shapehaving a plurality of vane-shaped mold sections 10 e″ disposed about theperiphery a central hub mold section 10 f″. The mold section 10 d″ has athin wall 10 w″ with a relatively small wall thickness as describedabove to define mold cavity MC″ in the mold sections 10 e″ and 10 f″.The pour cup 10 a″, collar 10 b″, and sprue or runner 10″ can have alarger wall thickness than that of the mold section 10 d″. The largerwall thickness of the pour cup 10 a″, collar 10 b″, and sprue or runner10 c″ is provided by additional dips of ceramic during the shell buildprocess.

For purposes of illustration and not limitation, the wall thickness ofthe mold section 10 d″ typically is in the range described above for theembodiment of FIGS. 1-2A, 2B. The ceramic shell mold 10″ can be formedof one or more appropriate ceramic or refractory materials, orcombinations thereof, such as including but not limited to zirconia,silica, alumina, graphite, and other ceramic materials selected independence on the particular molten metal or alloy to be cast asdescribe above.

After the ceramic inner shell mold region 10″ is formed, the fugitivemetallic outer backup mold region 20″ is formed thereon. Oneillustrative embodiment of the invention shown in FIGS. 6-7 involvesplacing a portion of the sprue or runner 10 c″ and the mold section 10d″ of the shell mold region 10″ in a cylindrical or other shaped metal(e.g. aluminum) pan 31″ and introducing molten backup material (e.g.molten tin) in the mold cavity MM″ defined between the inner mold region10″ and the pan 31″ to a level just below the upper lip 31 a″ of the pan31″ and solidifying the molten backup material in-situ on the innershell mold region 10″.

The composite mold 50″ pursuant to this illustrative embodiment of theinvention thereby comprises the inner mold region 10″ and the fugitivemetallic outer backup mold region 20″ on the inner mold region with orwithout the pan 31″, FIG. 7.

The outer backup mold region 20″ is relatively thick compared to theinner mold region 10″ for the reasons described above in connection withFIG. 1-2A, 2B. For purposes of illustration and not limitation, the wallthickness of the outer backup mold region 20″ typically is in the rangeof 0.25 to 1.0 inch when applied on the above-described thin wall innermold region 10″ made by the “lost wax” process, although the thicknessof the outer backup mold region 20″ may vary from location to locationdepending on the exterior configuration of the inner mold region 10″.The invention is not limited to any particular thickness of the outerbackup mold region 20″ as described above.

FIG. 8 illustrates placement of the composite mold 50″ pursuant to theanother illustrative embodiment of the invention in a modified diecasting machine. The die casting machine of FIG. 8 is similar to thatshown in FIG. 3 such that like features of the casting machines bearlike reference numerals. The casting machines are similar with theexception that the casting machine of FIG. 8 differs in having a metal(e.g. steel) gas impermeable container 60″ between platens 14, 16 forreceiving the composite mold 50″. The container 60″ comprises a tubularbody 60 a″ which can be circular, square or any other cross-sectionalshape. The container 60″ includes a welded-on end closure 62″. Thecontainer 60″ also includes a removable end closure 64″ fastened towelded-on annular flange 66″ by fasteners 67″ in a manner to define aninternal container chamber 68″ in which the mold 50″ is received forcasting with the space about the mold 50″ filled with refractoryparticulates 71″. The removable end closure 64″ engages an O-ring vacuumseal S1″for establishing a vacuum tight seal between the end closure 64″and the annular flange 66″ of the container 60″ when the end closure 64″is fastened on the container using fasteners 67″. An O-ring vacuum sealS2″ is provided between the end closure 62″ and the base plate 30 forestablishing a vacuum tight seal therebetween when the container 60″ isabutted to the base plate 30 as will be described below. The container60″ is not permanently fastened to the base plate 30. The vacuum sealsS1″, S2″ may comprise Viton material or other suitable high temperaturesealing material.

The container end closure 62″ includes a passage that is adapted toreceive the shot sleeve 24 in flow relationship to the pour cup 10 a″ ofthe composite mold 50″ when the mold 50″ is clamped in position in thecontainer 60″. In particular, the container 60″ includes internallythereof a plurality of elongated clamping fingers 60 f″ that aretightened over the surface of the collar 10 b″ to clamp the mold 50″ infixed position against end closure 62″ in the chamber 68″. The clampfingers 60 f″ are tightened using fasteners 70″ which are threaded intothe end closure 62″.

When so clamped, the pour cup 10 a″ and the sprue 10 c″ of the compositemold 50″ communicates to the shot sleeve 24 for receiving moltenmetallic material from the shot sleeve 24 as pushed by the plunger 27.The shot sleeve 24 is sealingly received in passage of end closure 62″.The collar 10 b″ seals against leakage of molten metallic material fromthe shot sleeve and the mold pour cup. The container 60″ includesnipples 60 n″ to permit loose (free flowing) refractory particulates71″, such as alumina or zirconia ceramic back-up sand, about the mold50″ in the container 60″. The nipples 60 n″ then are closed by fittingsto prevent the loose particulates 71″ from falling out. The container60″ also includes hoist rings 60 r″ to allow use of a conventionaloverhead hoist to lift and transport the container 60″ for mold loadingand unloading purposes.

In practicing a method embodiment of the invention, a solid ingot of themetallic material to be die cast is charged into the crucible 54 in thevacuum melting chamber 40. The container 60″ with the composite mold 50″therein is held between the base plate 30 and the platen 16 by movementof platen 16 relative to platen 14. The container 60″ is filled with theloose back-up particulates 71″ via nipples 60 n″ before the container60″ is held in position relative to the fixed platen 14 by movement ofthe movable platen 16. The loose particulates 71″ are introducedmanually or by machine through the nipples 60 n″ into the chamber 68″and about the mold 50″.

The vacuum chamber 40 then is evacuated to a suitable level for meltingthe particular charge (e.g. less than 100 microns for titanium alloyssuch as Ti-6A1-4V alloy and titanium aluminide such as TiAl) by vacuumpump P. The container 60″ with the composite mold 50″ therein heldbetween the base plate 30 and the platen 16 is concurrently evacuated to'the same vacuum level through the connection to the vacuum meltingchamber 40 via the shot sleeve 24 and by virtue of being isolated fromsurrounding ambient air atmosphere by the container vacuum seals S1″,S2″.

The composite mold 50″ typically is at ambient (room) temperature whenit is placed in the container 60″. Alternately, the mold 50″ can bepreheated to a suitable elevated temperature before being placed in thecontainer 60″ or while it is in the container using cartridge orresistance heaters.

The solid charge of the metal or alloy in crucible 54 is melted byenergizing induction coil 56, the melt then is poured under vacuum intothe shot sleeve 24 via the melt inlet 50 with the plunger 27 initiallypositioned at the start injection position of FIG. 8. The molten metalor alloy is poured into the shot sleeve 24 and resides therein for apreselected dwell time to insure that no molten metal gets behind theplunger 27. The melt can be poured directly from the crucible 54 viainlet 50 into the shot sleeve 24, thereby reducing time and metalcooling before injection can begin.

The plunger 27 then is advanced in the shot sleeve 24 by actuator 25 toinject the molten metal or alloy under hydraulic pressure through thegating system and through the mold pour cup 10 a″ and sprue 10 c″ intothe mold cavity MC″ of the composite mold 50″. The plunger 27 isadvanced by a hydraulic system described in copending patent applicationSer. No. 11/311,910 incorporated herein by reference to this end.

The molten metal or alloy is forced at velocities, such as 10-120 inchesper second for titanium alloys and titanium aluminides, down the shotsleeve 24 and into the evacuated molds 50′.

After the molten metal or alloy is at least partially solidified,typically mostly solidified, in the mold 50″, the fugitive metallicouter mold region 20″ melts and drains away from the inner mold region10″ to the lower region of the steel container as result of extractionof heat from the cast and solidified metal or alloy through the innermold region 10″.

After the molten metal or alloy has been at least partially solidified,the base plate 30 and platen 16 are opened by movement of platen 16 awayfrom platen 14 within a typical time period that can range from 5 to 30seconds following injection to provide enough time for the molten metalor alloy to form at least a solidified surface on the cast component(s)in the mold 50″.

The container 60″ then is removed from the base plate 30 and transportedby a hoist's engaging hoist rings 60 r″ to an unloading station wherethe back-up particulates 71″ are removed by opening nipples 60 n″. Then,the end closure 64″ is removed so that the inner mold region 10″ can beunclamped and removed from the container 60″. The metallic materialsolidified in the inner mold region 10″ typically is substantiallysolidified by the time the inner mold region 10″ is removed from thecontainer. The inner mold region 10″ then is removed from the castcomponents by conventional techniques forming no part of the invention.The castings then can be inspected visually and by techniques accordingto customer requirements.

In die casting titanium alloys, titanium aluminide, nickel basesuperalloys, and cobalt based superalloys, the shot sleeve 24 andplunger tip 27 a contacting the molten metal can be made of anyappropriate material as described above.

Moreover, the particular casting parameters employed to cast a componentwill depend upon' several factors including mold size, gating, pourweight; and the fragility of the investment mold to the melt injectionpressures involved. The injection pressure is selected to retain thecomposite mold 50″ intact (no mold cracking or bursting under pressure)while achieving a satisfactory fill of the mold cavity regions. Thenominal weight of metal or alloy in the crucible 54 depends on the moldsize and the number of components to be die cast in the mold.

The following EXAMPLE 'is offered to further illustrate the inventionwithout limiting it.

EXAMPLE 2

Turbocharger wheels have been successfully die cast of titanium andtitanium aluminide (TiAl) alloys in composite molds of the type shown inFIG. 7 using a casting machine of the type shown in FIG. 8. Thecomposite mold comprised an inner mold region made of four dips of a waxpattern in a ceramic slurry followed by air drying and stuccoing toprovide a wall thickness of about 0.075 inch. An outer mold region oftin was applied to the inner mold region by casting as described aboveto a thickness of greater than 0.2 inch such that the molds had a shapesimilar to that shown in FIG. 7.

In general, the turbocharger wheels were made using casting parametersin the following ranges: melt injection pressure settings: 400-1800 psi;melt injection velocities (plunger speed): 10-120 in/sec; meltsuperheat: 0 to 75 degrees-F.; no mold preheat; shot sleeve length anddiameter: 15 inches and 3 inches; and limit switch 80 a set to dumpplunger fluid pressure when the plunger 27 is about 0.5 inch from itsfinal injection position. The melted tin previously forming the outerbackup mold region melted and drained from the inner mold regions bygravity for reuse.

Although the composite molds are illustrated above for making castturbocharger wheels, the invention is not so limited and can practicedto make other components that include, but are not limited to, internalcombustion engine valves, automotive or truck turbocharger compressorand turbine wheels, compressor and turbine blades and vanes for gasturbine engines, and medical components including hip stems, acetabularknees, tibial trays, and spinal components.

1. Composite mold for casting a metal or alloy, comprising an inner moldregion having a mold cavity and a fugitive metallic outer backup moldregion residing on the inner mold region, wherein metallic material ofthe backup mold region has such a melting temperature that the backupmold region melts from the inner mold region after the molten metal oralloy is cast and at least partially solidified.
 2. The mold of claim 1wherein the inner mold region comprises a shell mold.
 3. The mold ofclaim 2 wherein the shell mold has a wall thickness of about 0.1 inch orless.
 4. The mold of claim 1 wherein the backup mold region comprises ametal or alloy that melts at a temperature of about 1300 degrees F. orbelow.
 5. The mold of claim 4 wherein the backup mold region comprises ametallic material comprising tin, indium, bismuth, lead, aluminum, orzinc, or alloys thereof one with another or with other metals.
 6. Themold of claim 1 wherein the backup mold region comprises a metal oralloy that melts at a temperature of at least 500 degrees F. below thatof the metal or alloy to be cast in the mold.
 7. The mold of claim 1wherein the backup mold region is cast and solidified in-situ on theinner mold region.
 8. Composite mold for casting a metal or alloy,comprising a ceramic inner shell mold region having a mold cavity and afugitive metallic outer backup mold region residing on the inner shellmold region, wherein metallic material of the backup mold region hassuch a melting temperature less than about 1300 degrees F.
 9. The moldof claim 8 wherein the inner shell mold region has a wall thickness ofabout 0.1 inch or less.
 10. The mold of claim 8 wherein the backup moldregion comprises a metallic material comprising tin, indium, bismuth,lead, aluminum, or zinc, or alloys thereof one with another or withother metals.
 11. The mold of claim 8 wherein the backup mold region iscast and solidified in-situ on the inner mold region.
 12. Method ofcasting, comprising introducing a molten metal or alloy into a moldcavity of a composite mold having an inner mold region that includessaid mold cavity and a fugitive metallic outer backup mold regionresiding on the inner mold region, and melting the backup mold regionfrom the inner mold region after the molten metal or alloy is cast andat least partially solidified.
 13. The method of claim 12 wherein theinner mold region comprises a shell mold.
 14. The method of claim 13wherein the shell mold has a wall thickness of about 0.1 inch or less.15. The method of claim 12 wherein the backup mold region comprises ametal or alloy that melts at a temperature of about 1300 degrees F. orbelow.
 16. The method of claim 15 wherein the backup mold regioncomprises a metallic material comprising tin, indium, bismuth, lead,aluminum, or zinc, or alloys thereof one with another or with othermetals.
 17. The method of claim 12 including casting and solidifying thebackup mold region in-situ on the inner mold region before introducingthe molten metal or alloy.
 18. The method of claim 12 wherein the moltenmetal or alloy is introduced under pressure into the mold cavity. 19.The method of claim 18 wherein the molten metal or alloy is die castinto the mold cavity.
 20. The method of claim 12 wherein the moltenmetal or alloy is gravity cast into the mold cavity.
 21. Method ofcasting titanium aluminide, comprising introducing molten titaniumaluminide into a mold cavity of a composite mold having a ceramic innershell mold region that includes said mold cavity and a fugitive metallicouter backup mold region residing on the inner shell mold region, andmelting the backup mold region away from the inner shell mold regionafter the molten titanium aluminide is cast and at least partiallysolidified.
 22. The method of claim 21 wherein the inner shell mold hasa wall thickness of about 0.1 inch or less.
 23. The method of claim 23wherein the backup mold region comprises a metal or alloy that melts ata temperature of about 1300 degrees F. or below.
 24. The method of claim23 wherein the backup mold region comprises a metallic materialcomprising tin, indium, bismuth, lead, aluminum, or zinc, or alloysthereof one with another or with other metals.
 25. Method of making acasting mold, comprising forming a non-metallic refractory or ceramicinner shell mold region and forming a fugitive metallic outer backupmold region in-situ on the inner shell mold region.
 26. The method ofclaim 25 wherein the backup mold region is cast and solidified in-situon the inner mold region.